1. Field of the Invention
The present invention relates to an original image reading device for use in an image scanner, a facsimile machine and the like. More particularly, the invention relates to an original image reading device having a plurality of light receiving elements for receiving light reflected from an original document, which are formed on a substrate, and for outputing image data signals.
2. Description of the Related Art
With a conventional original image reading device, particularly a contact type image sensor, optical image data from the original is projected onto a light receiving element array without variation in size to be converted to electrical image data signal. In this case, the image projected is divided into a number of picture elements (corresponding to light receiving elements). Charges generated by the respective light receiving elements are temporarily stored every block in the capacitors existing among wires by using thin film transistor (TFT) switch elements, so as to read the stored charges in time-series manner out of the capacitors at a speed of several hundreds kHz to several hundreds MHz. The thus operated image sensor is known as a TFT drive type image sensor.
The TFT drive image sensor is operable for data reading with a single drive IC according to the operation of the TFT switch thereby resulting in reducing the number of drive ICs for driving the image sensor.
An example of an equivalent circuit of the TFT drive image sensor is as shown in FIG. 1, which is disclosed in Published Unexamined Japanese Patent Application No. Sho-63-9358 published in 1988. As shown, the TFT drive image sensor comprises a linear array 11 of light receiving elements, the length of which is approximately equal to the width of an original document, a charge transfer section 12 including thin film transistors T.sub.i,j (i=1 to N, and j=1 to n) connected to light receiving elements 11', respectively, and multi-layered wires 13 arrayed in a matrix fashion.
The linear array 11 is divided into N number of blocks each including n number of light receiving elements 11'. These light receiving elements 11' may be equivalently expressed in the form of photo diodes P.sub.j,i (j=1 to N, and j=1 to n). The light receiving elements 11' are connected to the drain electrodes of the thin film transistors T.sub.i,j, respectively. The source electrodes of the thin film transistors T.sub.i,j are connected, every block, through the matrix arrayed multi-layered wires 13 to n number of common signal lines 14. The common signal lines 14 are further connected to a drive IC 15.
The gate electrodes of the thin film transistors T.sub.i,j are connected to a gate pulse generator 16 so that the transistors are simultaneously conductive every block. Photocharges generated in the light receiving elements 11' are stored, for a fixed period of time, in the parasitic capacitors of the light receiving elements and the gate-drain overlap capacitors of the thin film transistors T.sub.i,j. Then, the photocharges, every block, are transferred to and stored in wire capacitors C.sub.i (i=1 to n) of the multi-layered wires 13. In this case, the thin film transistors T.sub.i,j operate as charge transfer switches.
In response to a gate pulse .phi.G1 from the gate pulse generator 16, the thin film transistors T.sub.l,l to T.sub.l,n of the first block are turned on, so that the charges generated and stored by the light receiving elements 11' are transferred to and stored in the wire capacitors C.sub.i. The charges stored in the wire capacitors Ci cause the potential of the common signal line 14 to vary. The varied potential is time sequentially output to the output line 17 by sequentially turning on the analog switches SW.sub.i (i=1 to n) within the drive IC 15.
Subsequently, in response to gate pulses .phi.G2 to .phi.Gn, the thin film transistors T.sub.2,1 to T.sub.2,n to T.sub.N,1 to T.sub.N,n of the second to Nth blocks are turned on. The charges generated and stored are transferred every block from the light receiving elements to the drive IC, and the charges are sequentially picked up. As a result, the image data signals of one scan line in the fast scan direction are collected. The above sequence of operations are repeated in the slow scan direction to thereby collect the image data signals of an image on the entire original document.
An overall construction of the original image reading device under discussion will be briefly described with reference to FIG. 2 which schematically illustrates the construction.
As shown in FIG. 2, the original image reading device includes a platen glass 21 on which an original document 20 is placed, a light source 22 for illuminating the original document 20 on the platen glass 21 with light emitted therefrom, a rod lens array 25 for condensing the reflected light from the original document 20 to form an erect image at 100% magnification on a light receiving element linear array 11, and the linear array 11 for converting the erect image of reflected light into electrical image data signals. The optical axis of the rod lens array 25 is perpendicular to the light receiving elements 11'.
With such a color contact type image sensor as disclosed in Published Unexamined Japanese Patent Application No. sho-63-76567 published in 1988, in which a color separation filter is attached to the light receiving part of light receiving elements, the same location on the original must be scanned plural times to collect color image data resulting in requiring much time to read the color image. More adversely, it is disadvantageous in that the processing for composing the image data that is read is complicated.
To increase the resolution, the conventional original image reading device employs the method to physically decrease the size of light receiving elements, thereby to increase a pixel (picture element) density of the light receiving elements array. However, there is a limit in decreasing the size of light receiving elements. This hinders the improvement of the resolution.
Additionally, since the focal depth of the rod lens array is small, problem arises when the original on the platen glass rise to the surface. As the result of the increased distance of the original from the rod lens array owing to the waving of the original to the surface , the rod lens array focuses the risen part of the image on the original at a location out of the focal depth. The risen part of the erect image is not focused on the linear array of light receiving elements, and visually become indistinct. Accordingly, high definition image data cannot be obtained.